The server doesn't have proper mime types set up for the new ms file types
add the following to the /etc/mime.types file and restart apache and the problem should be fixed.
# Added by myname 2009-06-03 application/vnd.ms-word.document.macroEnabled.12 docm application/vnd.openxmlformats-officedocument.wordprocessingml.document docx application/vnd.openxmlformats-officedocument.wordprocessingml.template dotx application/vnd.ms-powerpoint.template.macroEnabled.12 potm application/vnd.openxmlformats-officedocument.presentationml.template potx application/vnd.ms-powerpoint.addin.macroEnabled.12 ppam application/vnd.ms-powerpoint.slideshow.macroEnabled.12 ppsm application/vnd.openxmlformats-officedocument.presentationml.slideshow ppsx application/vnd.ms-powerpoint.presentation.macroEnabled.12 pptm application/vnd.openxmlformats-officedocument.presentationml.presentation pptx application/vnd.ms-excel.addin.macroEnabled.12 xlam application/vnd.ms-excel.sheet.binary.macroEnabled.12 xlsb application/vnd.ms-excel.sheet.macroEnabled.12 xlsm application/vnd.openxmlformats-officedocument.spreadsheetml.sheet xlsx application/vnd.ms-excel.template.macroEnabled.12 xltm application/vnd.openxmlformats-officedocument.spreadsheetml.template xltx
REFERENCES
http://serverfault.com/questions/19060/why-are-docx-xlsx-pptx-downloading-from-webserver-as-zip-files
